The aqueous urea solutions containing non-reacted reactants (NH.sub.3 and ammonium carbamate), as well as the solutions of carbamate itself, are strongly corrosive, at the high temperatures, with respect to the metal surfaces lapped by such liquids, and, in spite of all the technological expedients performed until now, heavy corrosion problems must still be faced, particularly in the stripping devices, typical of the modern high temperature and high pressure processes for the manufacture of urea. In general, the most showy corrosion phenomena occur in the uppermost portion of said strippers, where the most part of the heat is supplied and where the evaporation of the non-reacted reactants reaches its highest point, and the traditional addition of oxygen (also in considerable amounts), as a passivation agent, into the bottom of the strippers, does not allow to avoid such phenomena.
The passivation by means of gaseous agents--for instance air--allows to reach a satisfactory level of protection on the surfaces wetted by a non-evaporating liquid and in all parts exposed to condensing vapors; such passivation however does not protect completely the critical points of the modern urea processes, which use strippers consisting of falling-film heat-exchangers. The best known of such processes is the so-called I.D.R. process described in Pagani U.S. Pat. No. 4,208,347.
Belgian Pat. No. 625,397 teaches how to passivate the internal surfaces of a reactor for the synthesis of urea, consisting of a stainless steel containing up to 19% by weight of Cr and 14% by weight of Ni, and describes the use of oxygen, as a passivating agent, at 180.degree. C. and 270 Kg/cm.sup.2, while suggesting generically to replace oxygen by other passivating agent, e.g. hydrogen peroxide, alkali metal peroxides and alkaline earth peroxides.